One example of an image display device that displays an input video signal on a display panel such as a liquid crystal panel is disclosed in Japanese Publication for Unexamined Patent Application No. 65528/1999 (Tokukaihei 11-65528, published on Mar. 9, 1999).
The image display device disclosed in this publication processes an input video signal and displays it on a liquid crystal panel. In addition, the light source projects light on the liquid crystal panel according to the input video signal.
Referring to FIG. 8, a schematic structure of an image display device 80 of the foregoing publication is described below.
The image display device 80 includes a low-pass filter 81, a peak value detector 82, a high-frequency cut filter 83, a lighting circuit 84, a light source 85, a video signal processing circuit 86, a driver 87, and a liquid crystal panel 88.
The low-pass filter 81 removes a high-frequency component of the video signal 89 it receives, and outputs a low-pass filter output signal 90.
The peak value detector 82 detects and holds a peak value of the low-pass filter output signal 90, and outputs a peak value signal 91.
As will be described later, the image display device 80 according to the foregoing configuration successively displays images on a frame basis. Accordingly, the “peak value” is meant to indicate, for example, a pixel-wise peak value of the video signal in one frame with respect to each color component.
The peak value detector 82 has a function known as a peak-hold function, and the detected peak value is attenuated with a time constant of substantially one frame. In this way, a peak value can be detected per frame.
The high-frequency cut filter 83 outputs a filter output signal 92, which is produced by slowing the change of the peak value signal 91, to the lighting circuit 84 and the video signal processing circuit 86.
In the liquid crystal panel 88, the time required for the display compensation is comparatively longer than that required for the compensation of light intensity of the light source 85. It is for this reason that the high-frequency cut filter 83 slows the signal change, so that the operation of display compensation in the liquid crystal panel 88 can properly follow the change in light intensity of the light source 85.
Another reason the high-frequency cut filter 83 slows the rate of change of light intensity of the light source 85 is to prevent display flicker, which occurs when the intensity of the light from the light source 85 changes abruptly.
The lighting circuit 84 processes the filter output signal 92 by pulse width modulation control for example, and outputs a lighting circuit output signal 93 to the light source 85. The light source 85 projects light onto the liquid crystal panel 88 with the intensity according to the lighting signal output signal 93.
The video signal processing circuit 86 processes the video signal 89 according to the filter output signal 92, and outputs a video signal processing circuit output signal 94 to the driver 87.
The driver 87 displays an image on the liquid crystal panel 88 according to the video signal processing circuit output signal 94.
In response to the video signal 89 via the low-pass filter 81 and the video signal processing circuit 86, the image display device 80 according to the foregoing configuration operates in the manner described below. Namely, the image display device 80 processes the video signal 89 and displays it on the liquid crystal panel 88. In addition, the image display device 80 lights the light source 85 according to the video signal 89, so as to project light onto the liquid crystal panel 88.
More specifically, in response to the video signal 89, the image display device 80 according to the foregoing configuration processes the video signal 89 by the described operations of the respective constituting elements, so that an image is displayed on the liquid crystal panel 88 by the driver 87.
Here, the driver 87 of the image display device 80 displays the video signal with respect to each pixel of the liquid crystal panel 88, so as to display a complete image over the entire screen.
The image display device 80 displays an image of one frame over a predetermined time period in the described manner, and the next frame is displayed based on the next input of the video signal. By repeating this process, a desired image (moving image) is displayed on the liquid crystal panel 88.
The output value of the light source 85 is updated according to the video signal every time the liquid crystal panel 88 is refreshed per frame.
Referring to FIG. 9, switching operations of the video signal and the lighting control signal will be described below.
FIG. 9 shows a timing chart, in which the horizontal axis indicates time and the vertical axis indicates signal intensity. The periods T11 through T15 on the upper part of FIG. 9 indicate time intervals.
A signal 95 is one example of the peak value signal 91 shown in FIG. 8. The periods T11, T13, and T15 indicate time periods in which the video signal has a large peak value with respect to each color component, i.e., the periods of bright display. On the other hand, the periods T12 and T14 indicate time periods in which the video signal has a small peak value with respect to each color component, i.e., the periods of dark display.
As FIG. 9 indicates, the signal 95 as one example of the peak value signal 91 changes over time according to the level of brightness of the video signal.
A signal 96 of FIG. 9 in the example of the signal 95 corresponds to the filter output signal 92 of FIG. 8. Namely, the signal 96 is produced by processing the signal 95 in the high-frequency cut filter 83, the signal 95 being one example of the peak value signal 91. The signal 95 (96) is supplied to the lighting circuit 84 to produce therein the driving signal 93 for the light source 85.
A signal 97 of FIG. 9 in the example of the signal 95 corresponds to the video signal processing circuit output signal 94 of FIG. 8. That is, the signal 97 is a signal that is produced by processing the signal 95, which is one example of the peak value signal 91, in the high-frequency cut filter 83 and subsequently in the video signal processing circuit 86. The signal so produced is supplied to the driver 87, so as to output a driving signal to the liquid crystal panel 88.
Signals 98 and 99 shown in FIG. 9 are signals that are produced when the effects of the high-frequency cut filter 83 are enhanced.
That is, the signal 98 of FIG. 9 in the example of the signal 95 corresponds to the filter output signal 92 of FIG. 8, and it enhances the effects of the high-frequency cut filter 83.
The signal 99 of FIG. 9 in the example of the signal 95 corresponds to the video signal processing circuit output signal 94 of FIG. 8, and it enhances the effects of the high-frequency cut filter 83.
By thus using the high-frequency cut filter 83, changes in illumination intensity and changes in video signal do not cause a significant change in a display state of the liquid crystal panel. As a result, flicker can be prevented.
Referring to FIG. 9, the following considers the case where the display changes from bright to dark at the transition of T11 to T12, for example.
In this case, the signal 95, which is a peak value signal, changes from High level to Low level. The signal 96 also changes from High level to Low level, while the signal 97, which is a video signal processing circuit output signal, changes from Low level to High level.
On the other hand, when the display changes from dark to bright, for example, at the transition of T12 to T13, the signals 95 and 96 changes from Low level to High level, while the signal 97 changes from High level to Low level.
The signals 98 and 99, which are produced when enhancing the effects of the high-frequency cut filter 83, show a similar pattern of change as the signals 96 and 97. As described, in the image display device 80, the signal 96 for controlling the lighting circuit is also changed at the timing of change of the peak value signal 95 from bright display to dark display, or from dark display to bright display.
However, in the image display device 80, the same lighting control is employed for the display change of from dark to bright and from bright to dark. This increases the likelihood of display flicker when the display changes, for example, from bright display to dark display.
That is, in the image display device 80, the high-frequency cut filter 83 carries out the same control for the display change of from dark to bright and from bright to dark.
Accordingly, the filter output signal 92 changes at the same rate when the display changes from dark to bright and from bright to dark. The lighting circuit output signal 93 processed in the lighting circuit 84 also shows the same change.
As a result, the displayed image does not change smoothly, and, when a change in quantity of light is too large, flicker is caused, and in other cases, the “bright black display” phenomenon, in which a black scene appears too bright, may be caused in the display.
This is due to saturation of human visual characteristics with respect to changes in gradation and illuminance level. In a display change from dark display to bright display and from bright display to dark display, the degree of change perceived by the observer will not be the same even if the display change is controlled under the same conditions. For example, display flicker may be caused when the same control is carried out for the display change from bright to dark and from dark to bright. In this case, the displayed image does not match the human visual characteristics, and the observer fails to recognize the image properly.